pull to damp

I was looking at a cutaway of the old 1970's Rover 2000 car and saw something about the suspension that I had never noticed ( despite having owned one!). The car had both a clever de dion rear end and a front suspension with forward facing upper links. These were mounted high on the scuttle and operated the springs via bellcranks. That put all the loads direct into the scuttle but would it had little room for damper length and the dampers would have been horiziontial.

So the dampers were mounted down on the chassis rail and worked in the opposite way to normal - the damper extended with upward suspension movement and compressed with down movement.

Now I cannot see any benefit in this and I an see one minor disadvantage in that the damper is being compressed so flexing the valves against the tube with the more heavily damped return stroke.

Can anybody see any advantage of this "pull " layout and are there any examples in the racing world?

I will strongly disagree, the rear DeDion was a shocking piece of over-engineered rubbish, you couldn't make something more complicated if you set out to do so - all to be able to save on not having splined axle shafts.

The lower "trailing arm(s)" have a pivot not clearly seen here under the axle and the "sliding joint" both plunged and rotated - how this ever got into production is a complete mystery and Alfa Romeo would no doubt have used it as an exact "this is how NOT to do it" when they designed the wonderfully simple, light and extremely effective Alfetta DeDion.

Can anybody see any advantage of this "pull " layout and are there any examples in the racing world?

Many Buell Motorcycle models used a pull shock because it's a natural for a motorcycle allowing it to be mounted horizontally under the engine and simple pull linkage.

Harley used that in their Softail design as well. I've always been curious as to why other manufacturers didn't purse that simpler layout. In fairness however, the Softail rides like a hay wagon made of spaghetti. Two-up it might as well have only a rubber stop and no springs as it sits on the stop anyway.

A modern De Dion could be a more sensible thing, using a twistbeam as the wheel carrier unit. The part count is still almost as high as an IRS and there are still some kinematic nasties to work around. Rover was a truly odd, Citreon like, company, albeit with rather less Gallic flair, and rather more bicycle shop engineering. By the time I joined Spen King and Gordon Bashford were very much in charge of my division.

Anyway for the front suspension, way back when, those two were obsessed with modal performance (did you know that each car on the production line was individually modally tuned using an adjustable mass?), and quite obviously the firewall is the perfect place to react your loads, given the choice.

I can see no real reason or disadvantage for preferrring the shock one round or the other, there may be an internal arrangement that is slightly better. I suppose in a ride sense having the gas preload working for you, helping to support the car, is slightly preferable to having it work against you.

If you look at the Rover 3500 which replaced the 2000and was a much simpler car it had a solid axle with side radius arms and a torque tube.

Very normal layout but the spring mounts were sort of cantilevered out from the side radius arms they rested on.

Greg, have I got that right?

Not on the picture I have which shows the spring resting on the axle, the usual practice. That's in Bastow. What you propose would be tricky since all the vertical loads would be taken in coning of the arm bushes. Not impossible, but tricky.

Quite a superb suspension design for the prevailing road conditions of the time [and its anticipated market]

http://en.wikipedia....iki/Citroën_2CV"The Suspension of the 2CV was almost comically soft; a person could easily rock the car side to side dramatically (back and forth was quite a bit more resistant). The leading arm / trailing arm swinging arm, fore-aft linked suspension system together with inboard front brakes had a much smaller unsprung weight than existing coil spring or leaf spring designs. It was designed by Marcel Chinon.[6]The system comprises two suspension cylinders mounted horizontally on each side of the platform chassis. Inside the cylinders are two springs, one for each wheel, mounted at each end of the cylinder. The springs are connected to the front leading swinging arm and rear trailing swinging arm, that act like bellcranks by pull rods (tie rods). These are connected to spring seating cups in the middle of the cylinder, each spring being compressed independently, against the ends of the cylinder. Pictured in reference.[6][36][37]If each cylinder was rigidly mounted to the chassis, it would provide fully independent suspension, but it is not rigidly mounted. It is mounted using an additional set of springs, originally made from steel, called "volute" springs, but on later models made from rubber. These springs allow the front and rear suspension to interconnect.[6]When the front wheel is deflected up over a bump, the front pull rod compresses the front spring inside the cylinder, against the front of the cylinder. This also compresses the front "volute" spring pulling the whole cylinder forwards. That action pushes the rear wheel down on the same side via the rear spring assembly and pull rod. When the rear wheel meets that bump a moment later, it does the same in reverse, keeping the car level front to rear. When both springs are compressed on one side when travelling around a bend, or front and rear wheels hit bumps simultaneously, the equal and opposite forces applied to the front and rear spring assemblies reduce the interconnection significantly, or even completely.[3] This stiffens the suspension after a certain amount of body roll has been achieved. It allows the 2CV to have very soft "bump mode" absorption, without wallow or uncontrolled float.[6]It reduces pitching, which is a particular problem of soft car suspension.[3]At high angles of body roll, the swinging arms that are mounted with large bearings to "cross tubes" that run side to side across the chassis; combined with the effects of all-independent soft springing and excellent damping, this keeps the road wheels in contact with the road surface and parallel to each other across the axles. A larger than conventional steering castor angle, ensures that the front wheels are closer to vertical than the rears, when cornering hard with a lot of body roll. All this provides excellent road holding, while appearing to look like a softly sprung American car with poor handling and road holding because of poor body control. The other key factor in the quality of its road holding is the very low and forward centre of gravity, provided by the position of the engine and transmission.[6]The suspension also automatically accommodates differing payloads in the car- with four people and cargo on board the wheelbase increases by around 4 cm (2 in) as the suspension deflects, and the castor angle of the front wheels increases by as much as 8 degrees thus ensuring that ride quality, handling and road holding is almost unaffected by the additional weight.[6]On early cars friction dampers (like a dry version of a multi-plate clutch design) were fitted at the mountings of the front and rear swinging arms to the cross-tubes. Because the rear brakes were outboard, they had extra tuned mass dampers to damp wheel bounce from the extra unsprung mass. Later models had tuned mass dampers at the front (because the leading arm had more inertia and "bump/thump" than the trailing arm), with hydraulic telescopic dampers / shock absorbers front and rear. The uprated hydraulic damping obviated the need for the rear inertia dampers.[6] (It should be noted that only dampers designed to be able to work horizontally should be used as replacements. Some that will physically fit do not work properly horizontally.)It was designed to be a comfortable ride by matching the frequencies encountered in human bipedal motion.[6]This sophisticated suspension design ensured the road wheels followed ground contours underneath them closely, while insulating the vehicle from shocks, enabling the 2CV to be driven over a ploughed field as its design brief required. More importantly it could comfortably and safely drive at reasonable speed, along the ill-maintained and war-damaged post war French Routes Nationales. It was commonly driven 'Pied au Plancher' - 'foot to the floor' by their peasant owners.[3][38]The 2CV suspension was assessed by Alec Issigonis and Alex Moulton in the mid-1950s (according to an interview by Moulton with CAR magazine in the late 1990s); this inspired them to design the Hydrolastic suspension system for the Mini and Austin 1100, to try to keep the benefits of the 2CV system but with added roll stiffness in a simplified design."

Not on the picture I have which shows the spring resting on the axle, the usual practice. That's in Bastow. What you propose would be tricky since all the vertical loads would be taken in coning of the arm bushes. Not impossible, but tricky.

Greg, my apologies, I was trying to remember something I read 40 years ago!

It was the dampers that were cantilevered off the axle not the radius arrms to improve roll control on the 3500, not the springs.

Here is the article as it appeared

It was in copy of Motor magazine ( when such technicalities were covered), it may be wrong but you are a better judge of that than me

That makes more sense. To be honest I think that that is one of those drawing board arguments, yes you have a rationale, but who says equal is best? Most production cars are underdamped in roll when correctly damped for pitch and bounce.

That makes more sense. To be honest I think that that is one of those drawing board arguments, yes you have a rationale, but who says equal is best? Most production cars are underdamped in roll when correctly damped for pitch and bounce.

Greg, I've long wondered about the ratio of roll to heave/pitch damping. It has always seemed to me that cars are significantly underdamped in roll.....I can only assume that proper roll damping levels would result in overdamped heave and pitch mtions.

Yup, hence the interest in Kinetic's approach, and other systems that allow you to look at roll as an input to damper force. If you damped a car 'correctly' in roll it would bounce off rough roads, at a guess.

Primary ride is easy to get more or less right with the basic equations, but usefully modelling shock absorber behaviour is the holy grail of my job, and I don't think I have ever helped anybody with that. Tires are sexier but we actually have some reasonable (one thing at a time, that is I can model wet handling or dry handling or impacts or etc etc. not everything at once) models of them, and if push comes to shove we do know how to model them properly, but it is very, very expensive (pretty much like running crash simulations). Shocks haven't had the effort put in, and in some respects they are a trickier proposition.

Greg, I've long wondered about the ratio of roll to heave/pitch damping. It has always seemed to me that cars are significantly underdamped in roll....

Well silly manufacturers keep putting those wheel things on the cars and they get in the way of the ideal shock/spring position forcing them to be mounted inboard.

Leverage 101, if you mounted your springs smack in the center of an axle (lets stick with live axle for simplicity) you would have zero roll resistance and as you work your way outwards with the mounts you increase the roll resistance. If you could mount your springs on outriggers outside of the wheels you could have high roll resistance with soft ride.

Of course maxay and Greg were talking about roll damping not springs but same principles apply. Its not that hard to devise mechanisms that combine high roll spring rate or damping with low heave rates. (ARB's for example).

Of course maxay and Greg were talking about roll damping not springs but same principles apply. Its not that hard to devise mechanisms that combine high roll spring rate or damping with low heave rates. (ARB's for example).

Different spring frequencies tend to cancel each other out (when working together) the same reason you use double valve springs in an engine.

Sorry Cheapy but thats another urban myth. As with spring rate, the differing frequencies combine to produce a new effective frequency. What is reduced, is the input from internal resonances in the spring(s) (in helical springs but not really relevent to torsion bars including ARBs). This cancellation is one reason for multiple valve springs (because they may need to be operated at the (quite high) internal resonant frequencies). The other is actual damping, due to friction between the coils of the two (or more) springs.

Yes, but what would be big deal to add rotary dampers to ARB... Mechanical, which I would presume to be relatively low-cost, like used on Auto-Unions in '30ies would seem up to the task because they could be adjusted by simply tightening them (and I think one would have to take care of low speed damping only). I think old monoshock systems IMHO had this failing because they didn't seem to have damping in roll, with dampers working only in high-speed area of their characteristic; and I feel this could have been addressed by fitting dampers to ARB.

The other is actual damping, due to friction between the coils of the two (or more) springs.

That's the urban myth actually, sure they dampen (a proper damper spring that is, not just 2 springs rubbing against each other) but with such inconsistency, that in itself causes issues to the point the big boys stopped doing it years ago.

My Valiant ute of some many years ago with eff'ed shocks would argue about torsion bars having a lack of frequency - a coil spring is after all a curved torsion bar.

Yes, but what would be big deal to add rotary dampers to ARB... Mechanical, which I would presume to be relatively low-cost, like used on Auto-Unions in '30ies would seem up to the task because they could be adjusted by simply tightening them (and I think one would have to take care of low speed damping only).

The trick is that it would have to be low cost AND not screw anything else up. NVH, durability and especially ride. Friction is the enemy of good quality ride, a friction damper of almost any form is not acceptable.

A good question is, how much sta bar would we use if it was able to be damped? I think on-road SUVs would benefit, but then they'd fall over more often, so that doesn't seem like a good idea.

That's the urban myth actually, sure they dampen (a proper damper spring that is, not just 2 springs rubbing against each other) but with such inconsistency, that in itself causes issues to the point the big boys stopped doing it years ago.

My Valiant ute of some many years ago with eff'ed shocks would argue about torsion bars having a lack of frequency - a coil spring is after all a curved torsion bar.

There is a big difference between the natural frequency of a spring mass system (that you are referring to) and internal resonance frequencies of a helical spring (that I was referring to). Internal resonance is the mass of the coils themselves causing oscillations, often observed as "ripples" travelling from end to end in the spring. Valve spring slo-mo is a place to observe the phenomenon. Internal resonance for torsional springs is very different. The "mass" within the spring sees very little displacement - even with large force (torque) inputs. Consequently the internal resonant frequency is very high (orders of magnitude higher than an equivalent helical spring) and therefore not usually of any consequence for most applications.

I can't say for sure on double valve spring friction damping. You may be right but some sources still disagree with you. eghttp://www.hotrod.co...gs/viewall.htmlQUOTE:In the same vein, the classic dual spring with damper configuration is giving way to a dual spring in which the two elements have an interference fit.

The trick is that it would have to be low cost AND not screw anything else up. NVH, durability and especially ride. Friction is the enemy of good quality ride, a friction damper of almost any form is not acceptable.

A good question is, how much sta bar would we use if it was able to be damped? I think on-road SUVs would benefit, but then they'd fall over more often, so that doesn't seem like a good idea.

Further, I can't see the addition of a rotary damper to an ARB as straightforward. Basically you need to damp one end of the ARB wrt the other. Apart from the mechanical complexity, you would need a tricky damper to avoid the damper body adding to the unsprung mass of one wheel only.

Gruntguru, the type I had in mind actually did that (but originally those were main suspension springs), as shown in picture. Basically, I *ass*umed similar principle would be applicable to ARBs... Admittedly, I thought (Coulomb) friction could be used for damping, as in case of A-U, but will accept Greg's verdict that it's a bad thing.

Rover 2000 .... front suspension with forward facing upper links. These were mounted high on the scuttle and operated the springs via bellcranks.

I think I recall reading this configuration was to allow the possible use of Moulton Hydrolastic suspension units, (rubber spring and fluid damper combined) I don't see an obvious way that Hydrolastic could be used with the deDion rear axle. Can anyone shed any light on this?

There was a rather nice Rover engined special which had another novel de-Dion arrangement, the drive shafts are fixed length as is the de-Dion tube. This works by using open roller bearings at the hubs to allow the axle shafts to slide in their bearings and thus track variation. (Thereby relieving the de-Dion tube and its longitudinal location from all lateral load)

I think I recall reading this configuration was to allow the possible use of Moulton Hydrolastic suspension units, (rubber spring and fluid damper combined) I don't see an obvious way that Hydrolastic could be used with the deDion rear axle. Can anyone shed any light on this?

Hydrolastic and Rover were opposing teams, I can't see that there would have been any chance of hydro being used on a Rover at that time.

Since this thread became a discussion on roll damping and with the parallel thread on no warp set ups I have been trying to see a way of empircally testing the separation of roll and bump damping.

I have come up with an idea which could be tested, I think, on a beam rear axle or de dion car so I thought I would run it by here. Sophisticated it is not I warn you.

Basically you fit a longitudinal bolt at the rear of the axle casing at the desired roll centre with a standard Watts linkage attached to it. The only difference is that the bolt protudes further back than the central Watts pivot plate and that plate extends vertically as far as possible beyond the upper Watts lateral link mounting point. The vertical extension has a series of mounting holes in it.

A damper is mounted on the extension of the Watts pivot bolt on the axle so it has only bump control but nil roll control. Then a vertical bracket is mounted on the axle to one side and a horiziontal damper is fitted between that axle bracket and the vertical extension of the Watts pivot plate.

Since the pivot plate remains ( esentially ) fixed versus the chassis by the lateral links and the axle is pivoting around the plate this damper provides full but pure roll damping.

So find a bumpy wide road and set out some cones then run a series of slaloms with various leverage damper settings. By having holes in both the vertical Watts plate and the axle vertical plate you can quickly adjust the damper leverage in roll without changing the valving settings.

I.m not sure if this work be valid experimental set up but it is quite simple to implement

You're a bit late, member who is currently posting, Johan Lekas, is building that very principle in his unusual bike engined twin beam spaceframed thingy right now...

Maybe Johan can update us if possible please?

The only drawback I could see with this principle is that he'd possibly need 'non-conventional' dampers to avoid different damping characteristics in left and right turns (I think 'normal' car dampers should have difference between bump and rebound damping by some 3x)...

You're a bit late, member who is currently posting, Johan Lekas, is building that very principle in his unusual bike engined twin beam spaceframed thingy right now...

Maybe Johan can update us if possible please?

Yepp. This is what it looks like. It's built according to the principle I described in the other thread. (DaveW got me a bit nervous that I have missed something crucial, but I think it's OK. will post an answer in that thread)Engine is Aprilia RSV 1000. Telescopic steering column with the R&P on the axle

You appear to have 50/50 split roll resistance currently, why didn't you make the quadrants with multiple hole choices to easily; a/ change roll bias and b/ change roll stiffness?

You are quite right, in principle. Controlling the roll (moment) distribution provides a method of controlling lateral balance in a turn. A roll moment distribution can be expressed as a combination of roll & warp, and it is the warp component that provides balance control. So far as I'm aware, there are three main methods of controlling roll moment bias in a turn; one is to have different (adjustable, ideally) roll centre heights, another is to control warp load directly & the last to controll the torque distribution (last added for completness).

In a conventional suspension a change in roll moment distribution is achieved by roll stiffness distribution, which yields the required warp load because the suspension has a warp stiffness. In a suspension that truely has zero warp stiffness, changing roll stiffness distribution will not (I think) change warp load, because the chassis will simply move to null the effect. What is required in a vehicle that doesn't roll, or has zero warp stiffness, is a mechanism to provide the stabilzing turn dependent warp offset.

Having different roll cente heights would be one way of achieving this. Active suspension provided a way of achieving a low warp stiffness with a directly controllable warp offset. The Lotus system, for example, embodied an algorithm that used the steering input and vehicle responses as a "trajectory" command - effectively making the vehicle neutral, at least whilst all four wheels remained in contact.

There are slots in one of the arms of the rockers for adjustment (visible in the front axle picture)

You also seem to have multiple holes (3?) where the linkage attaches to the top of the rear beam.

I like it! It is indeed equivalent to the Erik Z. sketch I posted in the KDSS thread. Zero warp stiffness!

Roll centres are quite low. Did you consider making them adjustable? Of course your roll resistance and damping is fully adjustable. Are you confident that roll damper has identical push and pull characteristics?

In a conventional suspension a change in roll moment distribution is achieved by roll stiffness distribution, which yields the required warp load because the suspension has a warp stiffness. In a suspension that truely has zero warp stiffness, changing roll stiffness distribution will not (I think) change warp load, because the chassis will simply move to null the effect.

I think it will. The roll moment will be shared by the axles in proportion to the linkage ratios. Imagine Johan's roll damper locked solid. The now rigid F/R connecting mechanism becomes part of the lateral location for each axle and is almost equivalent to a change in RCH with that change differing F-R depending on the linkage ratios.

Perhaps a better way to look at it is another extreme example. Move the Z-Bar linkage at one end down close the RC. That end will now have almost zero roll stiffness and lateral weight transfer. The other end will have roll stiffness and weight transfer governed by the roll spring/damper.